Method and Apparatus for Increasing Current Capacity of a Distributed Drive System

ABSTRACT

An improved system for connecting a DC bus cable and a remote motor drive includes a capacitance module and an extension module that may each be mounted adjacent to the remote motor drive. The capacitance module includes a first DC bus connector and a second DC bus connector. The first DC bus connector includes a terminal block configured to receive a pair of conductors for the DC bus. The first DC bus connector further includes a pair of intermediate bus bars where each of the intermediate bus bars are connected at a first end to the terminal block and at a second end to a circuit board contained within the capacitance module. Traces on the circuit board are routed between the second ends of the intermediate bus bars and the second DC bus connector. The second DC bus connector is configured to be connected to DC bus bars.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 14/947,267, filed Nov. 20, 2015, which, in turn,claims priority to Indian patent application No. 1357/DEL/2015, filedMay 14, 2015, the entire contents of each application is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to a system forincreasing the current capacity of a distributed motor drive system and,more specifically, to a capacitance module and an extension modulemountable proximate to an inverter in the distributed motor drivesystem.

As is known to those skilled in the art, a motor drive is used toconvert energy from a first form to a second form. The input voltage maybe a DC voltage having a fixed amplitude or a single phase ormulti-phase AC voltage having a fixed amplitude and frequency. Theoutput voltage may be an AC voltage having a variable frequency and/oramplitude, where the motor drive controls the frequency and amplitudesupplied to the motor such that the motor rotates at a desired speed orproduces a desired torque. According to one common configuration, themotor drive includes a front end, configured to convert an AC inputvoltage to a DC voltage present on a DC bus, and an inverter, configuredto convert the DC voltage from the DC bus to the AC voltage provided tothe motor.

Some controlled systems, such as a process line or a machining center,may include multiple motors, each controlling a different axis ofmotion. The controlled system receives AC power from the utility grid,which is, in turn distributed to each of the motors. It may beadvantageous to provide a single AC-to-DC converter, or rectifier,having a power rating sufficient to provide power for all of the motorsin the controlled system, to convert the AC power to DC power fordistribution via a shared DC bus. Although the single rectifier modulemay require more space due to its power rating than a rectifier sizedfor one of the motors, the overall space required may be reduced by notrequiring a separate rectifier for each motor. Each motor may then havean associated motor drive which includes an inverter connected to theshared DC bus. The motor drive controls operation of the inverter toprovide AC power to the motor to achieve a desired operation of themotor. Each motor drive may be mounted proximate to the rectifier moduleor, optionally, may be distributed about the controlled system proximateto the motor being controlled by the motor drive.

However, shared DC bus systems have not been fully met without incurringvarious disadvantages. Because the DC bus is shared between inverters,the conductors, or at least a portion of the conductors, for the DC busmust be sized to handle the total current capacity of all invertersconnected to the DC bus. If an inverter is located adjacent to therectifier module, a DC bus bar may be used to connect the two modules.The bus bar is a preformed, rigid conductor having a fixed routing path,often linear, between the two modules and is often rigidly connected toeach module. Such connection may be suitable for adjacent modules. If,however, an inverter is located remotely from the rectifier module, ittypically requires a DC bus cable. A DC bus cable may be routed in aflexible manner as required between modules. Increased current capacityin a DC bus cable is obtained by utilizing a larger wire gauge. However,as the wire gauge for the DC bus cable increases, the weight of thecable similarly increases and flexibility for routing is reduced. Theincreased weight and reduced flexibility of the cable generatesincreased strain on the connector for the motor drive to which the DCbus cable is connected. The physical limitations on connecting the DCbus cable to the motor drive may restrict the current rating for ashared DC bus and, therefore, restrict the number of motor drives thatmay be connected to the shared DC bus.

Thus, it would be desirable to provide an improved connector between aDC bus cable and motor drive to provide increased current capacity onthe shared DC bus.

As is known in the art, motor drives typically include a capacitanceconnected in parallel across the DC bus. The capacitance helps reducethe magnitude of ripple on the DC bus resulting from converting the ACvoltage to a DC voltage and helps maintain a constant amplitude ofvoltage on the DC bus. The capacitance additionally acts to reduceripple voltage on the DC bus resulting from the controlled switchingwithin the inverter to convert the DC voltage to the desired AC voltagewaveform. In a motor drive that includes both the rectifier and theinverter, each motor drive also includes a DC bus capacitance typicallywithin the motor drive. However, in the shared DC bus system, themajority of the DC bus capacitance may be located in the rectifiermodule. The individual motor drives typically have a nominal capacitanceand rely primarily on the capacitance in the rectifier module to reducethe ripple voltage on the DC bus resulting from the switching in eachmotor drive.

As previously indicated, when a motor drive is located remotely from therectifier module, a DC bus cable typically connects the shared DC busbetween the rectifier module and the motor drive. However, inductance inthe DC bus cable isolates the motor drive from the capacitance in therectifier module. As the distance of the DC bus cable increases, theinductance in the DC bus cable similarly increases. The increasedinductance creates greater isolation of the motor drives from thecapacitance in the rectifier module. As a result, an increase in ripplecurrent and/or harmonic content at the switching frequency or multiplesof the switching frequency for the motor drive is present on the sharedDC bus. These undesirable current components limit the amplitude ofdesirable DC bus current that may be conducted on the DC bus cablewithout exceeding the current rating for the DC bus cable and willreduce the life of the capacitors connected to the DC bus, potentiallyresulting in premature failure of a motor drive.

Thus, it would be desirable to provide a system that reduced the rippleand/or harmonic current present on the DC bus as a result of inverterswitching.

BRIEF DESCRIPTION OF THE INVENTION

The subject matter disclosed herein describes an improved system forconnecting a DC bus cable to a module in a distributed motor drivesystem and further describes an improved system for reducing the rippleand/or harmonic current present on the DC bus as a result of inverterswitching. A capacitance module is provided that may be mounted adjacentto one or more remote motor drives. The capacitance module includes ahousing, a first DC bus connector, and a second DC bus connector. Aportion of the first DC bus connector either extends outside the housingor is accessible via an opening in the housing. The portion of the firstDC bus connector accessible from outside the housing includes a terminalblock with wire terminals configured to receive conductors in a DC buscable. According to one embodiment of the invention, the terminal blockincludes a pair of wire terminals and each wire terminal is a mountingbolt. The DC bus cable includes two conductors, one for the positiverail and one for the negative rail of the DC bus. Each conductor isterminated with a lug configured to be secured to one of the mountingbolts. The first DC bus connector further includes a pair ofintermediate bus bars where each of the intermediate bus bars areconnected at a first end to one of the mounting bolts and at a secondend to a circuit board contained within the housing of the capacitancemodule. Traces on the circuit board are routed between the second endsof the intermediate bus bars and the second DC bus connector. The secondDC bus connector includes slots configured to receive DC bus bars. DCbus bars are inserted into the slot and run between the capacitancemodule and the one or more remote motor drives adjacent to thecapacitance module. The first DC bus connector may also include asupport member. The support member provides additional stability betweenthe mounting bolts and the circuit board to resist rotational and/orlateral forces applied to the mounting bolts, for example, whilesecuring the lugs to the mounting bolts or from the weight of the DC buscable. A capacitance is connected in parallel to the traces on thecircuit board providing additional bus capacitance at the remote motordrives.

According to another aspect of the invention, an extension module may bemounted adjacent to the capacitance module. The extension module alsoincludes a housing, a first DC bus connector and a second DC busconnector. The first and second DC bus connectors are the same as thefirst and second DC bus connectors on the capacitance module. A pair offlexible DC bus links is mounted between each wire terminal on the firstDC bus connector of the capacitance module and the corresponding wireterminal on the first DC bus connector of the extension module. The DCbus cable is then mounted to the extension module. DC bus bars areinserted into the slots of both the capacitance module and the extensionmodule as well as slots on DC bus connectors of each adjacent motordrive. The extension module, therefore, provides a parallel conductionpath with the capacitance module between the DC bus cable and the DC busbars and doubles the current capacity of the connection between the DCbus cable and the DC bus bars with the capacitance module alone.

According to one embodiment of the invention, a system for increasingthe current capacity of a shared DC bus is disclosed. The shared DC busincludes a set of DC bus wires and a set of DC bus bars and both the setof DC bus wires and the set of DC bus bars are operable to conduct a DCbus voltage. The system includes a bus power supply, at least oneinverter module, and a capacitance module. The bus power supply includesa first bus capacitance and a first set of terminals. The bus powersupply is operable to provide the DC bus voltage to the shared DC busvia the first set of terminals. At least one inverter module is locatedremotely from the bus power supply, and each inverter module includes asecond bus capacitance and a second set of terminals. The second set ofterminals is configured to receive the set of DC bus bars. Thecapacitance module is configured to be connected in series between thebus power supply and the inverter module. The capacitance moduleincludes a third bus capacitance, an internal DC bus, a third set ofterminals, and a fourth set of terminals. The third set of terminals isconfigured to receive the set of DC bus wires, the internal DC bus iselectrically connected in series between the third set of terminals andthe fourth set of terminals, and the fourth set of terminals isconfigured to receive the set of DC bus bars.

According to another embodiment of the invention, an apparatus forincreasing the current capacity of a distributed motor drive system isdisclosed. The distributed motor drive system includes a bus powersupply operable to supply a DC bus voltage, at least one inverter modulelocated remotely from the bus power supply, a set of DC bus wiresoperable to conduct the DC bus voltage, and a set of DC bus barsoperable to conduct the DC bus voltage. The apparatus includes acapacitance module and an extension module. The capacitance module andthe extension module are configured to be connected in parallel witheach other and the parallel connection of the capacitance module and theextension module is configured to be connected in series between the buspower supply and the inverter module. The capacitance module includes afirst set of terminals configured to receive the DC bus voltage from theset of DC bus wires, a second set of terminals configured to provide theDC bus voltage to the set of DC bus bars, and an internal DC buselectrically connected in series between the first set of terminals andthe second set of terminals. The extension module includes a third setof terminals configured to receive the DC bus voltage from the set of DCbus wires, a fourth set of terminals configured to provide the DC busvoltage to the set of DC bus bars, and an internal DC bus electricallyconnected in series between the third set of terminals and the fourthset of terminals. The third set of terminals is electrically connectedto the first set of terminals and the fourth set of terminals iselectrically connected to the second set of terminals.

According to yet another embodiment of the invention a DC bus connectoris disclosed. The DC bus connector establishes an electrical connectionbetween a first DC bus wire and a first DC bus bar terminal and betweena second DC bus wire and a second DC bus bar terminal. Each of the firstand the second DC bus bar terminals is mounted to a circuit board. TheDC bus connector includes a first wire terminal configured to receivethe first DC bus wire, a second wire terminal configured to receive thesecond DC bus wire, and a longitudinal support member having a first endand a second end opposite the first end. The first wire terminal and thesecond wire terminal are mounted proximate to the first end of thelongitudinal support member, and the first and second DC bus barterminals are proximate the second end of the longitudinal supportmember. The DC bus connector also includes an electrically insulatingmember mounted to the longitudinal support member between the first wireterminal and the second wire terminal, a first intermediate bus bar, anda second intermediate bus bar. Each of the first and second intermediatebus bars has a first end and a second end. The first end of the firstintermediate bus bar is mounted to the longitudinal support member andto the first wire terminal, establishing an electrical connectionbetween the first wire terminal and the first intermediate bus bar, andthe second end of the first intermediate bus bar is mounted to thecircuit board and electrically connected to the first DC bus barterminal via a first trace on the circuit board. The first end of thesecond intermediate bus bar is mounted to the longitudinal supportmember and to the second wire terminal, establishing an electricalconnection between the second wire terminal and the second intermediatebus bar, and the second end of the second intermediate bus bar ismounted to the circuit board and electrically connected to the second DCbus bar terminal via a second trace on the circuit board.

These and other advantages and features of the invention will becomeapparent to those skilled in the art from the detailed description andthe accompanying drawings. It should be understood, however, that thedetailed description and accompanying drawings, while indicatingpreferred embodiments of the present invention, are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the subject matter disclosed herein areillustrated in the accompanying drawings in which like referencenumerals represent like parts throughout, and in which:

FIG. 1 is an isometric view of an exemplary multi-axis drive systemincorporating a capacitance module and an extension module according toone embodiment of the invention;

FIG. 2 is a partial top plan view of the exemplary multi-axis drivesystem, capacitance module, and extension module from FIG. 1;

FIG. 3 is a side elevation view of the capacitance module of FIG. 1;

FIG. 4 is an isometric view of a circuit board from the motor drive ofFIG. 1 with a DC bus stab and the bus connector mounted thereto;

FIG. 5 an isometric view of a bus bar assembly for the shared DC busaccording to one embodiment of the invention;

FIG. 6 is a block diagram representation of a first exemplarydistributed motor drive system incorporating one embodiment of theinvention;

FIG. 7 is a block diagram representation of a second exemplarydistributed motor drive system incorporating one embodiment of theinvention;

FIG. 8 is a block diagram representation of the first exemplarydistributed motor drive system incorporating another embodiment of theinvention;

FIG. 9 is a block diagram representation of the second exemplarydistributed motor drive system incorporating another embodiment of theinvention;

FIG. 10 is a schematic representation of the capacitance module and theextension module of FIG. 1;

FIG. 11 is a top plan view of a flexible bus link utilized in oneembodiment of the present invention;

FIG. 12 is partial left side view of a DC bus connector utilized in theextension module from FIG. 1;

FIG. 13 is partial right side view of the DC bus connector utilized inthe extension module from FIG. 1;

FIG. 14 is an exploded view of the DC bus connector of FIG. 12 from thetop, front, and right side of the DC bus connector; and

FIG. 15 is an exploded view of the DC bus connector of FIG. 12 from thebottom, rear, and right side of the DC bus connector.

In describing the various embodiments of the invention which areillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is understood thateach specific term includes all technical equivalents which operate in asimilar manner to accomplish a similar purpose. For example, the word“connected,” “attached,” or terms similar thereto are often used. Theyare not limited to direct connection but include connection throughother elements where such connection is recognized as being equivalentby those skilled in the art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning initially to FIGS. 1-3, an exemplary multi-axis drive system 10utilizing a shared DC bus 20 is illustrated. The multi-axis drive system10 includes multiple modules, such as motor drives 12, a capacitancemodule 16, and an extension module 18. Each motor drive 12 may beconfigured to control operation of one motor, or axis, of the multi-axisdrive system 10. Each motor drive 12 may include, for example, aninverter to convert a DC voltage to an AC voltage, a rectifier toconvert an AC voltage to a DC voltage, a converter to convert a DCvoltage at a first voltage potential to a DC voltage at a second voltagepotential, or a combination thereof. According to the illustratedembodiment, each motor drive 12 receives power from the shared DC bus 20and includes an inverter operable to convert the power received from theshared DC bus 20 to a desired AC voltage to control operation of a motor(not shown) connected to the motor drive 12.

The shared DC bus 20 is connected to a remote power source, such as aregenerative module 13 or a rectifier module 15 (see e.g., FIGS. 6-9)via a DC bus cable 30. The DC bus cable 30 includes a first conductor 32and a second conductor 34 contained within an insulated cable jacket 31or sheath. A first lug 36 is connected to the end of the first conductor32 and a second lug 38 is connected to the end of the second conductor34. Each lug 36, 38 may be connected to the respective conductor 32, 34by crimping, soldering, or any other suitable method. Each lug 36, 38includes a first mating member 37, 39 operable to engage a complementarymating member mounted to the extension module 18. According to theillustrated embodiment, the first mating member 37, 39 is a generallyplanar surface with a hole extending through the planar surface. Thecomplementary mating member is a threaded rod, such as a mounting bolt105 on a DC bus connector 100 (see e.g., FIG. 3). The hole in the planarsurface is placed over the mounting bolt and a nut is then threaded onto the mounting bolt to secure the lug 36, 38 to the DC bus connector100.

A connector system 22 for the multi-axis drive system 10 includes a busbar assembly 40 and a bus bar connector 27 on each motor drive 12. Theconnector system 22 allows for a fast, modular connection of the sharedDC bus 20 between modules 12, 16, 18. According to the illustratedembodiment, the bus bar connector 27 is mounted to the top surface 14 ofeach module 12, 16, 18. Optionally, the bus bar connector 27 may beconnected to another surface of the modules 12, 16, 18 such as thefront, rear, or lower surface as long the surfaces of adjacent modules12, 16, 18 on which the bus bar connector 27 is mounted aresubstantially in the same geometric plane. With reference also to FIG.4, the bus bar connector 27 includes at least one external slot 28 withan opening external to the module 12, 16, 18, where each external slot28 is configured to receive a bus bar 60 (see e.g., FIG. 5) within theslot 28. The bus bar connector 27 further includes at least one internalslot 29 with an opening internal to the module 12, 16, 18, where eachinternal slot 29 is configured to engage a DC bus stab 21 mounted to acircuit board 9 within the module 12, 16, 18.

The connector system 22 also includes a bus bar assembly 40 used toestablish an electrical connection between bus bar connectors 27 onadjacent module 12, 16, 18. Turning next to FIG. 5, a bus bar assembly40 according to one embodiment of the invention is illustrated. The busbar assembly 40 includes a housing 42 and at least one bus bar 60. Thehousing 42 is a generally “u” shaped member having an upper wall 43, apair of side walls 45, a first end 46, and a second end 47. A clipportion 50 is integrally formed in the housing 42 on each side and ateach of the first end 46 and the second end 47 of the housing 42. Eachside wall 45 is joined to the upper wall 43 along an upper edge 48 ofthe housing 42 and extends longitudinally between clip portions 50 ateach end 46, 47 of the housing 42. The clip portion 50 includes an arm51 pivotally mounted to and extending downward from the upper edge 48 ofthe housing 42. According to the illustrated embodiment, the arm 51 isintegrally formed with the housing 42 of a resilient material allowingthe lower edge of the arm 51 to pivot about the edge 48 and return toits original position. The inner face of the arm 51 includes a retainingtab 53 extending longitudinally along the inner face of the arm 51 andproximate to the lower edge of the arm 51. The retaining tab 53 projectsinward into the housing 42 and includes a beveled surface 54 slopedoutward and downward from the inner face of the retaining tab 53. Thebeveled surface 54 of the retaining tab 53 on the clip 50 iscomplementary to a beveled outer edge of a retaining member 26 on thebus bar connector 27. The housing 42 also includes at least one slot 58configured to receive the bus bars 60. The slots 58 establish a frictionfit between each bus bar 60 and the housing 42. The housing 42 mayfurther include one or more bus bar retaining members (not shown) thatextend around the periphery of the bus bar 60. The bus bar retainingmember further defines the slot 58 in which each bus bar 60 is received.Separate bus bar retaining members may be located proximate to each end46, 47 of the housing 42 or a single bus bar retaining member may becentrally located within, or extend substantially the length of, thehousing 42.

The illustrated embodiment of the bus bar assembly 40 includes two busbars 60. Optionally, the bus bar assembly 40 may include four bus bars60 or other numbers of bus bars 60 corresponding to the number ofexternal slots 28 on the bus bar connector 27. Each bus bar 60 is formedof a conductive material, such as copper. Each bus bar 60 has an uppersurface 61, a lower surface 62, and a pair of side surfaces 63 each ofwhich extends between opposite ends 64, defining a generally rectangularbar. The height of each side surface 63 is greater than the width of theupper surface 61 and the lower surface 62 and the bus bar 60 extendslongitudinally within the housing 42 substantially between the first end46 and the second end 47 of the housing 42. Each bus bar 60 may beinserted into one of the slots 58 of the housing 42 and be retained by afriction fit and/or and adhesive. Optionally, the housing 42 may bemolded over the bus bars 60. The bus bar assembly 40 is used toestablish an electrical connection between bus bar connectors 27 onadjacent modules 12, as shown in FIG. 1. A suitable connector system 22for use with various embodiments of the present invention is disclosedin U.S. patent application Ser. No. 14/046,458, which is owned byRockwell Automation Technologies, Inc. and which is hereby incorporatedby reference.

The connector system 22, described above, provides a fast, modularconnection of the shared DC bus 20 between modules 12, 16, 18 when themodules 12, 16, 18 are mounted adjacent to each other or spaced apartfrom each other as long as the bus bar connectors 27 are aligned witheach other. However, numerous configurations exist in which at least aportion of the modules are not mounted adjacent to each other or withbus bar connectors 27 aligned to each other. Referring next to FIGS. 6-9several such configurations are illustrated. In FIG. 6, a regenerativemodule 13 receives power from an AC source 11. The AC source 11 may be asingle phase or multi-phase AC source. The regenerative module 13includes a set of controlled switches such as solid state devicesincluding, but not limited to, thyristors, silicon controlled rectifiers(SCRs), or transistors to convert the AC power to a DC voltage for theshared DC bus 20. A controller in the regenerative module 13 controlsoperation of the switches to selectively connect each phase from the ACsource 11 to the shared DC bus 20 and, thereby, controls bi-directionalcurrent flow between the AC source 11 and the shared DC bus 20. Theregenerative module 13 further includes a DC bus capacitance connectedbetween the positive and negative rails of the shared DC bus 20.According to the embodiment illustrated in FIG. 6, a capacitance module16 and multiple motor drives 12 are located remotely from theregenerative module 13. The remote location may be, for example, withina control cabinet but displaced from the regenerative module 13, withinseparate control cabinets, or at a location proximate to a controlledaxis of motion on a controlled machine or process. In any event, a DCbus cable 30 connects the regenerative module 13 and the capacitancemodule 16. A first end of the DC bus cable 30 is connected to DC busterminals on the regenerative module 13, and a second end of the DC buscable 30 is mounted to the capacitance module 16. The capacitance module16 also includes a DC bus capacitance connected between the positive andnegative rails of the shared DC bus 20. As will be discussed in moredetail below, the capacitance module 16 includes a DC bus connector 100configured to receive the second end of the DC bus cable 30 andestablishes an electrical connection between the DC bus cable 30 and theconnector system 22. The connector system 22 conducts the DC power onthe shared DC bus 20 to each of the motor drives 12.

In FIG. 7, a rectifier module 15 receives power from the AC source 11.The AC source 11 may be a single phase or multi-phase AC source. Therectifier module 15 includes a set of diodes, forming a diode bridge,that rectifies the input power from the AC source 11 to a DC voltage onthe shared DC bus 20. The rectifier module 15 further includes a DC buscapacitance connected between the positive and negative rails of theshared DC bus 20 The rectifier module 15 conducts current only from theAC source 11 to the shared DC bus 20. According to the embodimentillustrated in FIG. 7, the rectifier module 15 provides DC power to afirst series of motor drives 12 via a first connector system 22 a. Asecond series of motor drives, however, are located remotely from therectifier module 15 and the first series of motor drives 12. The remotelocation may be, for example, a second row within a control cabinet, aseparate control cabinet, or a location proximate to a controlled axisof motion on a controlled machine or process. A first capacitance module16 a is connected at the end of the first series of motor drives 12, anda second capacitance module 16 b is connected at the start of the secondseries of motor drives 12. Each of the capacitance modules 16 a, 16 bincludes a DC bus capacitance connected between the positive andnegative rails of the shared DC bus 20. A DC bus cable 30 connects thefirst and the second capacitance modules 16 a, 16 b. Each of thecapacitance modules 16 includes a DC bus connector 100 and each end ofthe DC bus cable 30 is connected to the DC bus connector 100 on therespective capacitance module 16. The first capacitance module 16 aestablishes an electrical connection between the first connector system22 a on the first series of motor drives and the DC bus cable 30. Thesecond capacitance module 16 b establishes an electrical connectionbetween the DC bus cable 30 and the second connector system 22 b. Thesecond connector system 22 b conducts the DC power on the shared DC bus20 to each of the motor drives 12 in the second series of motor drives.

In FIG. 8, another embodiment of the invention is illustrated. Aregenerative module 13 again receives power from an AC source 11. Justas in the embodiment illustrated in FIG. 6, the AC source 11 may be asingle phase or multi-phase AC source. The regenerative module 13includes a set of controlled switches such as solid state devicesincluding, but not limited to, thyristors, silicon controlled rectifiers(SCRs), or transistors to convert the AC power to a DC voltage for theshared DC bus 20. A controller in the regenerative module 13 controlsoperation of the switches to selectively connect each phase from the ACsource 11 to the shared DC bus 20 and, thereby, controls bi-directionalcurrent flow between the AC source 11 and the shared DC bus 20.According to the embodiment illustrated in FIG. 8, however, an extensionmodule 18 is added in parallel with the capacitance module 16. Theextension module 18, capacitance module 16, and multiple motor drives 12are located remotely from the regenerative module 13. The remotelocation may be, for example, within a control cabinet but displacedfrom the regenerative module 13, within separate control cabinets, or ata location proximate to a controlled axis of motion on a controlledmachine or process. In any event, a DC bus cable 30 connects theregenerative module 13 and the capacitance module 16. A first end of theDC bus cable 30 is connected to DC bus terminals on the regenerativemodule 13, and a second end of the DC bus cable 30 is mounted to theextension module 18. The extension module 18 and the capacitance module16 both include a DC bus connector 100 configured to receive the secondend of the DC bus cable 30. The second end of the DC bus cable 30 isconnected to the DC bus connector 100 of the extension module 18, andthe two DC bus connectors 100 are connected by a flexible bus link 70(see e.g., FIG. 2). Thus, the DC power conducted via the shared DC bus20 is conducted from the DC bus cable 30 through both DC bus connectors100. Each DC bus connector 100 establishes an electrical connectionbetween the DC bus cable 30 and the connector system 22. The connectorsystem 22 conducts the DC power on the shared DC bus 20 to each of themotor drives 12.

In FIG. 9, a rectifier module 15 receives power from the AC source 11.Just as in the embodiment illustrated in FIG. 7, the AC source 11 may bea single phase or multi-phase AC source. The rectifier module 15includes a set of diodes, forming a diode bridge, that rectifies theinput power from the AC source 11 to a DC voltage on the shared DC bus20. The rectifier module 15 conducts current only from the AC source 11to the shared DC bus 20. The rectifier module 15 provides DC power to afirst series of motor drives 12 via a first connector system 22 a. Asecond series of motor drives are located remotely from the rectifiermodule 15 and the first series of motor drives 12. The remote locationmay be, for example, a second row within a control cabinet, a separatecontrol cabinet, or a location proximate to a controlled axis of motionon a controlled machine or process. According to the embodimentillustrated in FIG. 9, however, an extension module 18 is added inparallel with the capacitance module 16. A first capacitance module 16 aand a first extension module 18 a are connected at the end of the firstseries of motor drives 12. A second extension module 18 b and a secondcapacitance module 16 b are connected at the start of the second seriesof motor drives 12. Each of the capacitance modules 16 and extensionmodules 18 includes a DC bus connector 100, and the DC bus connectors100 on each capacitance module 16 a, 16 b is connected to the DC busconnector 100 of the respective extension module 18 a, 18 b by aflexible bus link 70. The DC power conducted on the first connectorsystem 22 a is conducted through the DC bus connector 100 on both thefirst capacitance module 16 a and the first extension module 18 a to theDC bus cable 30. Each end of the DC bus cable 30 is connected to the DCbus connector 100 on the first and second extension modules 18 a, 18 b.The power on the DC bus cable is then conducted to the second connectorsystem 22 b through the DC bus connector 100 on both the secondextension module 18 b and the second capacitance module 16 b. The secondconnector system 22 b conducts the DC power on the shared DC bus 20 toeach of the motor drives 12 in the second series of motor drives. Theabove described configurations from FIGS. 6-9 are exemplary and forillustration. It is contemplated that still other configurations of DCbus power supplies, capacitance modules 16, extension modules 18, andmotor drives 12 may be utilized without deviating from the scope of theinvention.

Referring next to FIG. 11, one embodiment of a flexible bus link 70includes a first lug 72 and a second lug 82 with a cable 90 extendingtherebetween. The first lug 72 has a generally planar surface 74 with anopening 76 extending through the middle. Similarly, the second lug 82has a generally planar surface 84 with an opening 86 extending throughthe middle. Each of the first and second lugs 72, 82 are solid,conductive members. The cable 90 may be a flat, braided cable. The cable90 has an insulating member 92 such as heat-shrink tubing enclosing thecable 90 between the lugs 72, 78. The cable 90 is connected to one edgeof each of the first and second lugs 72, 82 by crimping, soldering orany other suitable method to obtain an electrical connection between theeach lug 72, 82 and the cable 90. The opening 76, 86 in each lug 72, 82is configured to pass over the threaded member 105 of the DC busconnector 100. Each lug 72, 82 may then be retained to the DC busconnector 100 by threading a nut onto the threaded member 105 andcapturing the lug 72, 82 between the nut and the DC bus connector 100.

Turning next to FIGS. 12-15, a DC bus connector 100 according to oneembodiment of the invention is illustrated. The DC bus connector 100provides a physical connection for the DC bus cable 30 of the shared DCbus 20 and establishes an electrical connection between the DC bus cable30 and a circuit board 9 on which the bus bar connector 27 is mounted.The DC bus connector 100 includes a longitudinal support member 110having a first end 112 and a second end 114 opposite the first end 112.The longitudinal support member 110 also includes a lower surface 116,an upper surface 118, a right side 115, and a left side 117, each ofwhich extends between the first end 112 and the second end 114, defininga generally rectangular bar. A pair of openings 120 extends through thelongitudinal support member 110 proximate the first end 112 of thelongitudinal support member 110. The diameter of each opening 120 islarge enough to receive one of the mounting bolts 105 through theopening 120. Each mounting bolt 105 serves as one terminal on which oneof the lugs 36, 38 from the DC bus cable 30 may be mounted. According toanother embodiment of the invention, a pair of threaded rods may bemolded within the longitudinal member to define the terminals for the DCbus cable 30. Optionally, still other configurations of terminals may beutilized as long as one mating connection is provided on the DC busconnector 100 and the complementary mating connection is provided on theend of each conductor 32, 24 of the DC bus cable 30. The longitudinalsupport member is made from an electrically insulating material such asepoxy.

The DC bus connector 100 further includes a first intermediate bus bar130 and a second intermediate bus bar 140. The first intermediate busbar 130 includes a first end 132 and a second end 134. The first end 132of the first intermediate bus bar 130 includes a first planar mountingsection 131 with an opening 133 extending therethrough. The first planarmounting section 131 of the first intermediate bus bar 130 is mounted tothe upper surface 118 of the longitudinal support member 110 such thatthe opening 133 in the first planar mounting section 131 aligns with theopening 120 closest to the first end 112 of the longitudinal supportmember 110. A longitudinal segment 135 of the first intermediate bus bar130 is connected to the first planar mounting section 131 and wrapsaround to the left side 117 of the longitudinal support member 110. Thelongitudinal segment 135 is generally orthogonal to the first planarmounting section 131 and extends toward but not fully to the second end114 of the longitudinal support member 110. The longitudinal segment 135of the first intermediate bus bar 130 runs generally parallel with andadjacent to the circuit board 9 within the capacitance module 16 or theextension module 18 in which it is mounted. At least one mounting screw137 secures the longitudinal segment 135 of the first intermediate busbar 130 to the circuit board 9. According to the embodiment illustratedin FIG. 13, two mounting screws 137 secure the longitudinal segment 135to the circuit board 9. A trace on the circuit board 9 runs between thelocation at which the longitudinal segment 135 is connected and one ofthe DC bus stabs 21. The first intermediate bus bar 130 is formed froman electrically conductive material, such as copper and is preferablyformed, for example, by stamping from a single piece of copper. Thus,the first intermediate bus bar 130 establishes an electrical connectionbetween a first of the mounting bolts 105 and a first of the DC busstabs 21.

The second intermediate bus bar 140 includes a first end 142 and asecond end 144. The first end 142 of the second intermediate bus bar 140includes a first planar mounting section 141 with an opening 143extending therethrough. The first planar mounting section 141 of thesecond intermediate bus bar 140 is mounted to the upper surface 118 ofthe longitudinal support member 110 such that the opening 143 in thefirst planar mounting section 141 aligns with the opening 120 displacedfurthest from the first end 112 of the longitudinal support member 110.A longitudinal segment 145 of the second intermediate bus bar 140 isconnected to the first planar mounting section 141 and extends along theupper surface 118 of the longitudinal support member 110. Thelongitudinal segment 145 is generally coplanar with the first planarmounting section 141 and extends toward the second end 114 of thelongitudinal support member 110. A second planar mounting section 146for the second intermediate bus bar 140 extends generally orthogonal tothe longitudinal segment 145 proximate the second end 114 of thelongitudinal support member 110. The second planar mounting section 146for the second intermediate bus bar 140 extends generally parallel withand adjacent to the circuit board 9 within the capacitance module 16 orthe extension module 18 in which it is mounted. At least one mountingscrew 147 secures the second planar mounting section 146 of the secondintermediate bus bar 140 to the circuit board 9. According to theembodiment illustrated in FIG. 12, two mounting screws 147 secure thesecond planar mounting section 146 to the circuit board 9. A trace onthe circuit board 9 runs between the location at which the second planarmounting section 146 is connected and one of the DC bus stabs 21. Thesecond intermediate bus bar 140 is formed from an electricallyconductive material, such as copper and is preferably formed, forexample, by stamping from a single piece of copper. Thus, the secondintermediate bus bar 140 establishes an electrical connection between asecond of the mounting bolts 105 and a second of the DC bus stabs 21.

A central insulting member 150 is mounted to the upper surface 118 ofthe longitudinal support member 110. The central insulating member 150is mounted between the first planar sections 131, 141 of the first andsecond intermediate bus bars 130, 140 and extends in a generallyorthogonal direction from the upper surface 118 between each of themounting bolts 105. A first side insulating member 155 is mountedproximate the first end 112 of the upper surface 118 of the longitudinalsupport member 110 and on the opposite side of the first planar section131 of the first intermediate bus bar 130 from the central insulatingmember 150. The first side insulating member 155 extends in a directiongenerally orthogonal from the upper surface 118 of the longitudinalsupport member 110. A second side insulating member 160 is mounted alongthe upper surface 118 of the longitudinal support member 110 and on theopposite side of the first planar section 141 of the second intermediatebus bar 140 from the central insulating member 150. The second sideinsulating member 160 extends in a direction generally orthogonal fromthe upper surface 118 of the longitudinal support member 110. A spaceexists between a lower edge 161 of the second side insulating member andthe upper surface 118 of the longitudinal support member 110 ofsufficient thickness to allow the longitudinal segment 145 of the secondintermediate bus bar 140 to pass through. Each of the insulating members150, 155, 160 is made from an electrically insulating material, such asepoxy. The first side insulating member 155 and the central insulatingmember 150 are spaced a sufficient distance to permit the first lug 36of the DC bus cable 30 to fit between the two members. The second sideinsulating member 160 and the central insulating member 150 are spaced asufficient distance to permit the second lug 38 of the DC bus cable 30to fit between the two members.

The DC bus connector 100 also includes a support member 170 to resistrotational forces applied to the DC bus connector 100 from the DC buscable 30. According to the illustrated embodiment, an “s” shaped supportmember 170 is mounted to the longitudinal support member 110 via a pairof stand-offs 190. The support member 170 includes a first planarsurface 171 oriented parallel and adjacent to the circuit board 9. Apair of openings 172 in the first planar surface 171 allows the supportmember 170 to be secured to the circuit board 9 by, for example, screwsor bolts inserted through the openings 172 and through correspondingopenings in the circuit board 9. It is not intended for the first planarsurface 171 or for securing members, such as the screws or bolts, toprovide an electrical connection between the support member 170 and thecircuit board 9. The support member 170 may be made of an electricallyconductive material, such as a metal, or an electrically non-conductivematerial, such as an epoxy or plastic. The outer layer of the circuitboard 9 is a non-conductive substrate and traces on the circuit board 9are routed around areas in which the support member 170 contacts thecircuit board 9. The support member 170 further includes a second planarsurface 173 and a third planar surface 175. The second planar surface173 extends generally orthogonal to the first planar 171 surface awayfrom the circuit board 9. The third planar surface 175 extends generallyorthogonal to the second planar surface 173. A pair of standoffs 190 aremounted between the lower surface 116 of the longitudinal support member110 and the second planar surface 173, such that an axis 111 extendingthrough the longitudinal support member 110 would be offset from butgenerally parallel to an axis 177 extending through the second planarsurface 173. Optionally, various other configurations of support membersextending from the longitudinal support member 110 to the circuit board9 may be utilized as long as the support member 110 is configured toresist forces applied to the wire terminals 105 from the DC bus cable30.

Where upper and lower, front and rear, left and right, top and bottom,and other like terms denoting a direction are utilized herein, it isunderstood that the directions are relative to each other and do notdenote an absolute direction. A device may be mounted in a reverseorientation such that the directions are similarly reversed withoutdeviating from the scope of the invention.

In operation, the capacitance module 16 and the extension module 18 withtheir respective DC bus connectors 100 provide a system for increasingcurrent capacity in a system having a shared DC bus 20. As previouslyindicated, a shared DC bus 20 may utilize both a DC bus cable 30 and abus bar connector 27 to connect modules utilizing the shared DC bus 20.As the current rating of the shared DC bus 20 increases, the physicalsize of the DC bus cable 30 similarly increases in order to accommodatethe increased current capacity. As the DC bus cable 30 increases insize, its weight increases and flexibility decreases. Thus, mounting theDC bus cable 30 to a wire terminal 105 results in increased forceapplied to the wire terminal 105 both from the weight and from the DCbus cable 30 protruding more rigidly from the side of the wireterminals. The DC bus connector 100 as disclosed herein is configured tosupport the DC bus cable 30 being connected to the corresponding wireterminals 105. According to one embodiment of the invention, the DC busconnector is configured to receive a first and second DC bus conductors32, 24 rated to conduct 270 A.

A second problem also arises from an increase in the rating of the DCbus current. The current capacity between bus bar connectors 27 and thecircuit board 9 is limited by the size of the traces on the circuitboard 9. Increasing the current capacity of the bus bar connectors 27would require an increase in the size of the bus bar connector 27 and,in particular to the footprint of the connection between the bus barconnector 27 and the circuit board 9. The capacitance module 16 and theextension module 18, each provided with the DC bus connector 100,provides a modular system where the capacitance module 16 may beutilized by itself or the capacitance module 16 and the extension module18 may be utilized in a pair to double the current capacity of theshared DC bus 20.

With reference to FIG. 10, a DC power source 7, such as the regenerativemodule 13 or the rectifier module 15, provides DC power to the shared DCbus 20. A DC voltage potential is present between the positive rail 17and the negative rail 19 of the DC bus 20. The first and the second DCbus conductors 32, 34 define the positive and negative rails 17, 19 ofthe shared DC bus 20 between the DC power source 7 and the extensionmodule 18. Each of the first and the second DC bus conductors 32, 34 issized to conduct at least 270 A of DC current. The first and the secondDC bus conductors 32, 34 are connected to the wire terminals 105 of theDC bus connector 100 within the extension module 18. In addition, afirst bus link 70 is connected between the first wire terminal 105 ofthe extension module 18 and the first wire terminal 105 of thecapacitance module 16. A second bus link 70 is connected between thesecond wire terminal 105 of the extension module 18 and the second wireterminal 105 of the capacitance module 16. The current in the DC bus issplit between each of the extension module 18 and the capacitance module16 such that about 135 A of DC current is conducted through each module16, 18. The DC current is conducted through the either the first orsecond wire terminal 105, intermediate bus bar 130, 140, trace on thecircuit board 9, and bus bar connector 27, thereby establishing anelectrical connection between each DC bus conductor 32, 34 and one ofthe DC bus bars 60. The capacitors 8 within the capacitance module 16are connected in parallel across the traces on the circuit board 9 andare charged from the DC bus current conducted through the capacitancemodule. Bus bar assemblies 40 are inserted into each of the bus barconnectors 27 to conduct the DC bus current from the capacitance module16 and the extension module 18 to each of the adjacent drive modules 12.The bus bar assemblies 40 are configured to conduct the full 270 A onthe shared DC bus when the bus current output from each of thecapacitance module 16 and the extension module 18 are recombined ontothe shared DC bus 20.

It should be understood that the invention is not limited in itsapplication to the details of construction and arrangements of thecomponents set forth herein. The invention is capable of otherembodiments and of being practiced or carried out in various ways.Variations and modifications of the foregoing are within the scope ofthe present invention. It also being understood that the inventiondisclosed and defined herein extends to all alternative combinations oftwo or more of the individual features mentioned or evident from thetext and/or drawings. All of these different combinations constitutevarious alternative aspects of the present invention. The embodimentsdescribed herein explain the best modes known for practicing theinvention and will enable others skilled in the art to utilize theinvention

We claim:
 1. An apparatus for reducing ripple current on a shared DC busin a distributed motor drive system, wherein the distributed motor drivesystem includes a plurality of motor drives, each connected to theshared DC bus, and the DC bus includes a set of DC bus wires extendingbetween at least a first portion of the plurality of motor drives and asecond portion of the plurality of motor drives, the apparatuscomprising: a housing operative to be mounted adjacent to one of theplurality of motor drives; a first set of terminals accessible through afirst opening in the housing and configured to receive the DC busvoltage from the set of DC bus wires; a second set of terminalsaccessible through a second opening in the housing and configured toreceive a set of DC bus bars, wherein the set of DC bus bars areoperative to connect the second set of terminals to the adjacent motordrive; an internal DC bus electrically connected between the first setof terminals and the second set of terminals; and a plurality ofcapacitors electrically connected between a positive rail and a negativerail of the internal DC bus.
 2. The apparatus of claim 1 furthercomprising a DC bus connector defined in part by the first set ofterminals and the internal DC bus, wherein: the first set of terminalsincludes a first wire terminal configured to receive a first DC bus wirefrom the set of DC bus wires and a second wire terminal configured toreceive a second DC bus wire from the set of DC bus wires; the internalDC bus includes a first intermediate bus bar and a second intermediatebus bar; the second set of terminals are mounted to a circuit boardwithin the housing; and the DC bus connector further includes: alongitudinal support member having a first end and a second end oppositethe first end, wherein the first wire terminal and the second wireterminal are mounted proximate to the first end of the longitudinalsupport member and the second set of terminals are proximate the secondend of the longitudinal support member, and an electrically insulatingmember mounted to the longitudinal support member between the first wireterminal and the second wire terminal, wherein: each of the first andthe second intermediate bus bars has a first end and a second end, thefirst end of the first intermediate bus bar is mounted to thelongitudinal support member and to the first wire terminal, establishingan electrical connection between the first wire terminal and the firstintermediate bus bar, and the second end of the first intermediate busbar is mounted to the circuit board and electrically connected to afirst terminal selected from the second set of terminals via a firsttrace on the circuit board; the first end of the second intermediate busbar is mounted to the longitudinal support member and to the second wireterminal, establishing an electrical connection between the second wireterminal and the second intermediate bus bar, and the second end of thesecond intermediate bus bar is mounted to the circuit board andelectrically connected to a second terminal selected from the second setof terminals via a second trace on the circuit board.
 3. The apparatusof claim 2 wherein the longitudinal support member defines a first axisextending along a length of the longitudinal support member and whereinthe DC bus connector further comprises: an additional support membermounted to the circuit board, wherein the additional support memberdefines a second axis displaced from the first axis in a plane generallyparallel to the circuit board; and at least one standoff mounted betweenthe longitudinal support member and the additional support member. 4.The apparatus of claim 2 further comprising: a first insulated sidemember extending generally orthogonal to the longitudinal support memberadjacent the first wire terminal; and a second insulated side memberextending generally orthogonal to the longitudinal support memberadjacent the second wire terminal.
 5. The apparatus of claim 1 whereinthe housing is a first housing and the internal DC bus is a firstinternal DC bus, the apparatus further comprising: a second housingoperative to be mounted adjacent to the first housing; a third set ofterminals accessible through a third opening located in the secondhousing, the third set of terminals configured to receive the DC busvoltage from the set of DC bus wires via an extension cable extendingbetween the first set of terminals and the third set of terminals; afourth set of terminals accessible through a fourth opening located inthe second housing, the fourth set of terminals configured to receivethe set of DC bus bars; and a second internal DC bus electricallyconnected between the third set of terminals and the fourth set ofterminals.
 6. The apparatus of claim 5 wherein each motor drive ismounted to a panel, the first housing is mounted to the panel adjacentto one of the motor drives, and the second housing is mounted to thepanel adjacent to the first housing.
 7. The apparatus of claim 5 furthercomprising a DC bus connector mounted, at least in part, within thesecond housing, wherein: the DC bus connector is defined in part by thethird set of terminals and the second internal DC bus, the third set ofterminals includes a first wire terminal configured to receive a firstDC bus wire from the set of DC bus wires and a second wire terminalconfigured to receive a second DC bus wire from the set of DC bus wires;the second internal DC bus includes a first intermediate bus bar and asecond intermediate bus bar; the fourth set of terminals are mounted toa circuit board within the second housing; and the DC bus connectorfurther includes: a longitudinal support member having a first end and asecond end opposite the first end, wherein the first wire terminal andthe second wire terminal are mounted proximate to the first end of thelongitudinal support member and the fourth set of terminals areproximate the second end of the longitudinal support member, and anelectrically insulating member mounted to the longitudinal supportmember between the first wire terminal and the second wire terminal,wherein each of the first and the second intermediate bus bars has afirst end and a second end, the first end of the first intermediate busbar is mounted to the longitudinal support member and to the first wireterminal, establishing an electrical connection between the first wireterminal and the first intermediate bus bar, and the second end of thefirst intermediate bus bar is mounted to the circuit board andelectrically connected to a first terminal selected from the fourth setof terminals via a first trace on the circuit board; the first end ofthe second intermediate bus bar is mounted to the longitudinal supportmember and to the second wire terminal, establishing an electricalconnection between the second wire terminal and the second intermediatebus bar, and the second end of the second intermediate bus bar ismounted to the circuit board and electrically connected to a secondterminal selected from the fourth set of terminals via a second trace onthe circuit board.
 8. The apparatus of claim 7 wherein the longitudinalsupport member defines a first axis extending along a length of thelongitudinal support member and wherein the DC bus connector furthercomprises: an additional support member mounted to the circuit board,wherein the additional support member defines a second axis displacedfrom the first axis in a plane generally parallel to the circuit board;and at least one standoff mounted between the longitudinal supportmember and the additional support member.
 9. The apparatus of claim 7further comprising: a first insulated side member extending generallyorthogonal to the longitudinal support member adjacent the first wireterminal; and a second insulated side member extending generallyorthogonal to the longitudinal support member adjacent the second wireterminal.
 10. An apparatus for reducing ripple current on a shared DCbus in a distributed motor drive system, wherein the distributed motordrive system includes a plurality of motor drives, each connected to theshared DC bus, and the DC bus includes a set of DC bus wires extendingbetween at least a first portion of the plurality of motor drives and asecond portion of the plurality of motor drives, the apparatuscomprising: a first capacitance module mounted adjacent to the firstportion of the plurality of motor drives, the capacitance moduleincluding: a first set of terminals configured to receive the DC busvoltage via a first set of DC bus bars from a motor drive in the firstportion of the plurality of motor drives, a second set of terminalsconfigured to provide the DC bus voltage to the set of DC bus wires, anda first internal DC bus electrically connected in series between thefirst set of terminals and the second set of terminals; and a secondcapacitance module mounted adjacent to the second portion of theplurality of motor drives, the capacitance module including: a third setof terminals configured to receive the DC bus voltage from the set of DCbus wires, a fourth set of terminals configured to provide the DC busvoltage to a motor drive in the second portion of the plurality of motordrives via a second set of DC bus bars, and a second internal DC buselectrically connected in series between the third set of terminals andthe fourth set of terminals.
 11. The apparatus of claim 10 wherein: thefirst capacitance module includes a plurality of capacitors electricallyconnected between a first positive rail and a first negative rail of thefirst internal DC bus, and the second capacitance module includes aplurality of capacitors electrically connected between a second positiverail and a second negative rail of the second internal DC bus.
 12. Theapparatus of claim 11 further comprising: a first extension modulemounted adjacent to the first capacitance module, the first extensionmodule including: a fifth set of terminals configured to receive the DCbus voltage via the first set of DC bus bars, a sixth set of terminalsconfigured to receive the DC bus voltage from the first set of DC buswires via an extension cable extending between the first set ofterminals and the sixth set of terminals, and a third internal DC buselectrically connected between the fifth set of terminals and the sixthset of terminals.
 13. The apparatus of claim 12 wherein a first portionof the plurality of motor drive is mounted to a first panel, the firstcapacitance module is mounted to the first panel adjacent to one of thefirst portion of the plurality of motor drives, and the first extensionmodule is mounted to the first panel adjacent to the first capacitancemodule.
 14. The apparatus of claim 12 further comprising: a secondextension module mounted adjacent to the second capacitance module, thesecond extension module including: a seventh set of terminals configuredto receive the DC bus voltage from the set of DC bus wires andconfigured to receive an extension cable extending between the seventhset of terminals and the third set of terminals, an eighth set ofterminals configured to be connected in series with the fourth set ofterminals via the second set of DC bus bars, and a fourth internal DCbus electrically connected between the seventh set of terminals and theeighth set of terminals.
 15. The apparatus of claim 14 wherein: a firstportion of the plurality of motor drive is mounted to a first panel, thefirst capacitance module is mounted to the first panel adjacent to oneof the first portion of the plurality of motor drives, the firstextension module is mounted to the first panel adjacent to the firstcapacitance module, a second portion of the plurality of motor drive ismounted to a second panel, the second capacitance module is mounted tothe second panel adjacent to one of the second portion of the pluralityof motor drives, and the second extension module is mounted to thesecond panel adjacent to the second capacitance module.
 16. Theapparatus of claim 10 wherein each of the first capacitance module andthe second capacitance module include a DC bus connector, comprising: afirst wire terminal configured to receive a first DC bus wire from theset of DC bus wires and a second wire terminal configured to receive asecond DC bus wire from the set of DC bus wires, wherein the first wireterminal and the second wire terminal define one of the first set ofwire terminals and the third set of wire terminals; an additionalinternal DC bus including a first intermediate bus bar and a secondintermediate bus bar; a circuit board on which one of the second set ofterminals and the fourth set of terminals are mounted; and alongitudinal support member having: a first end and a second endopposite the first end, wherein the first wire terminal and the secondwire terminal are mounted proximate to the first end of the longitudinalsupport member and one of the second set of terminals and the fourth setof terminals are proximate the second end of the longitudinal supportmember, and an electrically insulating member mounted to thelongitudinal support member between the first wire terminal and thesecond wire terminal, wherein: each of the first and the secondintermediate bus bars has a first end and a second end, the first end ofthe first intermediate bus bar is mounted to the longitudinal supportmember and to the first wire terminal, establishing an electricalconnection between the first wire terminal and the first intermediatebus bar, and the second end of the first intermediate bus bar is mountedto the circuit board and electrically connected to a first terminalselected from one of the second set of terminals and the fourth set ofterminals via a first trace on the circuit board; the first end of thesecond intermediate bus bar is mounted to the longitudinal supportmember and to the second wire terminal, establishing an electricalconnection between the second wire terminal and the second intermediatebus bar, and the second end of the second intermediate bus bar ismounted to the circuit board and electrically connected to a secondterminal selected from one of the second set of terminals and the fourthset of terminals via a second trace on the circuit board.
 17. Theapparatus of claim 16 wherein the longitudinal support member defines afirst axis extending along a length of the longitudinal support memberand wherein the DC bus connector further comprises: an additionalsupport member mounted to the circuit board, wherein the additionalsupport member defines a second axis displaced from the first axis in aplane generally parallel to the circuit board; and at least one standoffmounted between the longitudinal support member and the additionalsupport member.
 18. The apparatus of claim 16 further comprising: afirst insulated side member extending generally orthogonal to thelongitudinal support member adjacent the first wire terminal; and asecond insulated side member extending generally orthogonal to thelongitudinal support member adjacent the second wire terminal.
 19. Theapparatus of claim 14 wherein each of the first extension module and thesecond extension module include a DC bus connector, comprising: a firstwire terminal configured to receive a first DC bus wire from the set ofDC bus wires and a second wire terminal configured to receive a secondDC bus wire from the set of DC bus wires, wherein the first wireterminal and the second wire terminal define one of the sixth set ofwire terminals and the eighth set of wire terminals; an additionalinternal DC bus including a first intermediate bus bar and a secondintermediate bus bar, a circuit board on which one of the fifth set ofterminals and the seventh set of terminals are mounted; and alongitudinal support member having: a first end and a second endopposite the first end, wherein the first wire terminal and the secondwire terminal are mounted proximate to the first end of the longitudinalsupport member and one of the fifth set of terminals and the seventh setof terminals are proximate the second end of the longitudinal supportmember, and an electrically insulating member mounted to thelongitudinal support member between the first wire terminal and thesecond wire terminal, wherein: each of the first and the secondintermediate bus bars has a first end and a second end, the first end ofthe first intermediate bus bar is mounted to the longitudinal supportmember and to the first wire terminal, establishing an electricalconnection between the first wire terminal and the first intermediatebus bar, and the second end of the first intermediate bus bar is mountedto the circuit board and electrically connected to a first terminalselected from one of the fifth set of terminals and the seventh set ofterminals via a first trace on the circuit board; the first end of thesecond intermediate bus bar is mounted to the longitudinal supportmember and to the second wire terminal, establishing an electricalconnection between the second wire terminal and the second intermediatebus bar, and the second end of the second intermediate bus bar ismounted to the circuit board and electrically connected to a secondterminal selected from one of the fifth set of terminals and the seventhset of terminals via a second trace on the circuit board.
 20. Theapparatus of claim 19 wherein the longitudinal support member defines afirst axis extending along a length of the longitudinal support memberand wherein the DC bus connector further comprises: an additionalsupport member mounted to the circuit board, wherein the additionalsupport member defines a second axis displaced from the first axis in aplane generally parallel to the circuit board; and at least one standoffmounted between the longitudinal support member and the additionalsupport member.